Mosfet switching circuit

Discussion in 'Digital Circuit Design' started by rashmihm, Dec 21, 2017.

  1. rashmihm

    Thread Starter New Member

    Sep 20, 2017
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    Hi,
    i am trying MOSFET switching circuit to switch milivolts voltage to volts but when i increase drain voltage resistor and MOSFET are heating what wold be the solution...
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    What do you mean by that?

    Could you upload the schematic as 800 x 600 .gif or jpg or png(for horizontal line and vertical line)?
    What are the input and output?
     
  3. rashmihm

    Thread Starter New Member

    Sep 20, 2017
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    I am getting millivolts but to drive transducer i need 3v to 5v Peak to Peak voltage.am using MOSFET switching circuit as shown in attachment.
    IMG20171227155043.jpg
     
  4. ScottWang

    Moderator

    Aug 23, 2012
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    If the input signal only has 750mV then you should use a npn transistor to amplify the signal and the output to drive the P type MOSFET, and if the output only needs 3~5Vpp then you will need a voltage divider.

    How much current you need from the output of MOSFET?

    Do you have more info about the transducer?
     
  5. Picbuster

    Active Member

    Dec 2, 2013
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    Look at the gate voltage needed to switch the fet on.
    My grey mass indicate 2 or 4 Volt but could be wrong.
    Note: obsolete part the eol was years ago.

    Picbuster
     
  6. rashmihm

    Thread Starter New Member

    Sep 20, 2017
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    1)I want to drive 1Mhz transducer, current has not been specified in data sheet but transmitter burst pulse should have 3Vp-p to 5Vp-p.
    2)when i provide 20v dc supply to the drain of MOSFET as shown in above circuit am getting output square wave of 2Vp-p but resister and transistor gets heated up.
    3)am not getting more then 750mv because am getting this voltage from TX pin of TDC-1000 chip.
     
  7. Bordodynov

    Well-Known Member

    May 20, 2015
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    See
    Draft613.png
     
  8. ScottWang

    Moderator

    Aug 23, 2012
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    The input signal is 750 mVpp do you think the MOSFET will be work?
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The mosfet will be biased by the voltage from the potmeter.
    The mosfet will be and stay in linear mode, so there will be heat generated.

    Bertus
     
  10. Bordodynov

    Well-Known Member

    May 20, 2015
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    I counted it. The power dissipated by the transistor turned out to be less than 0.68W.
     
  11. rashmihm

    Thread Starter New Member

    Sep 20, 2017
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    please suggest me an alternate circuit to boost 1Mhz pulses upto 5v peak to peak
     
  12. bertus

    Administrator

    Apr 5, 2008
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  13. Picbuster

    Active Member

    Dec 2, 2013
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    Enclosed a simple drawing.
    Make sure that the pfet is able to handle rise time (input impedance gate and its max voltage) .

    Picbuster
     
  14. ScottWang

    Moderator

    Aug 23, 2012
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    If the duty cycle of input signal is to narrow then it should be reduces the values of R2 and R3, the Imax ≅ 20mA, if you use the N MOSFET on the output then probably you can't get a 0V to 5V level signal.
    750mVpp to 5Vpp with 1Mhz_rashmihm_ScottWang-03.png

    Why you need to using 20V to be the power source, why not 12V or 5V?
     
  15. rashmihm

    Thread Starter New Member

    Sep 20, 2017
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    Hi scott,
    i tried with 5V and 12V for MOSFET circuit but it din't work out so i tried for 20v. Thanks for the circuit suggestion i will try out this and let u know the result.
     
  16. ScottWang

    Moderator

    Aug 23, 2012
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    If you using the circuit that your posted in the first post, the output voltage is quite high when the input is 0V then the output will be close to 20Vdc, for 12V is easy to make the MOSFET to turn on, but when you use 5V for the power source then you better to choose the type of Vgs has logic level = 4.5V or 5V.
     
  17. rashmihm

    Thread Starter New Member

    Sep 20, 2017
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    Hi scott,
    i tried the circuit without mosfet part that has attached but am not getting any wave form.
    1)Is it k if i use BC557 pnp transistor in place of 2n3906 pnp transistor
     
  18. ScottWang

    Moderator

    Aug 23, 2012
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    Try to measure the c of Q1, if it still no any waveform then the problem could be R1(reduces it) or the input voltage too less to active the Vbe as the same that the Ib1 is too small.
     
  19. rashmihm

    Thread Starter New Member

    Sep 20, 2017
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    I measured the voltage across collector of Q1,i got waveform but voltage has dropped am getting 58mv across the collector of Q1
     
  20. rashmihm

    Thread Starter New Member

    Sep 20, 2017
    12
    0
    I measured the voltage across collector of Q1 am getting waveform with 58mv there is a huge voltage drop after the transistor Q1 and across Q2 am not getting any waveform.
     
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